The present invention relates to a self-calibrating oscillator for automatically calibrating output frequency.
Resistance-capacitance (RC) oscillators are widely used in semiconductor products. This is because an RC oscillator is an inexpensive clock source and because various frequencies can be generated by varying the resistance and capacitance. However, a delay that occurs in the circuits of the oscillator may affect the oscillation frequency. The delay time is dependent on the power supply voltage, the environmental temperature, the manufacturing process, and the like. Thus, in practice, an RC oscillator is limited to low-frequency and low-accuracy applications.
Accordingly, an RC oscillator that generates oscillations with high accuracy has been developed (for example, refer to U.S. Pat. No. 5,594,388, page 1). The oscillator described in U.S. Pat. No. 5,594,388 uses a resistor and a capacitor, and has a time constant determined by the resistance and the capacitance. This oscillator differs from the conventional oscillators in that the RC circuit is not used for frequency generation. Instead, the RC time constant is compared to a period obtained from a frequency generated by another oscillation circuit so that self-calibration of the frequency of the entire RC oscillation circuit is performed in accordance with the comparison result.
The oscillator described in U.S. Pat. No. 5,594,388, however, needs a charge pump to generate a voltage signal for calibrating the oscillating frequency. In addition, the oscillator constantly self-calibrates the frequency and thus continuously consumes power.
It is an object of the present invention to provide a self-calibrating oscillator that efficiently increases the accuracy of an output frequency.